1. Field of the Invention
The present invention relates to a multicolor imaging system such as a photocopier, a facsimile machine, or a printer which comprises a feed back controller to rotate a belt at a constant rotation rate.
2. Description of the Related Art
Recently, in the field of an imaging system such as a photocopier or a printer, there has been an increasing demand for not only higher speed printing but also higher quality color image generation along with the widespread use of imaging devices such as a digital camera. In order to satisfy such a demand, a tandem type color imaging system including respective imaging units for yellow, cyan, magenta, and black has been widely used. This system is configured to transfer and superimpose four color toner images onto a transfer element or an intermediate transfer element in sequence to generate a color image in a single image generation process.
However, there is a problem in such a system including an intermediate transfer belt as a transfer element onto which toner images are transferred from four photoreceptor drums that a moving velocity of the intermediate transfer belt is changed due to eccentricity of a drive roller therefor or an error in engagement of drive gears, causing a color registration error in the toner images and degrading quality of generated images. Further, when the ambient environment of the imaging system changes or the inner temperature of the system changes due to a continuous paper feed, a belt drive roller may expand or contract and the average moving velocity of the transfer belt may change, which causes extension or reduction of toner images in a sub scan direction and a color registration error in the toner images as well as degrades the quality of color images.
In view of solving such problems, various techniques have been developed (disclosed in Japanese Laid-open Patent Publication No. 2004-220006 (Reference 1), Japanese Patent No. 3965357 (Reference 2), Japanese Laid-open Patent Publication No. 9-146329 (Reference 3), No. 2001-134039 (Reference 4), No. 2001-305820 (Reference 5), for example).
References 1 and 2 disclose a technique to control an intermediate transfer belt to move at a constant velocity by attaching a scale and a reader to the transfer belt or an encoder on a shaft of a driven roller moved with the transfer belt to accurately detect a moving velocity of the transfer belt and feed back detected velocity data to a drive motor. References 3 to 5 disclose a technique to adjust initial phases of rotations of four photoreceptor drums so that positional shifts of four toner images are coherent with one another on the intermediate transfer belt for the purpose of substantially reducing color registration errors caused by a velocity fluctuation in drive elements of each photoreceptor drum.
However, since in References 1 and 2 the intermediate transfer belt and the photoreceptor drums are driven by the same motor aiming for manufactural costdown, the transfer belt can be moved at a constant velocity by controlling the motor to eliminate the velocity fluctuation therein; however, it may cause a velocity fluctuation in the photoreceptor drums driven with the intermediate transfer belt. As a result, the rotary velocity of the photoreceptor drums is fluctuated by an amount caused by the velocity fluctuation of drum drive elements plus an amount caused by the velocity fluctuation of the motor.
The technique in References 3 to 5 has a problem that a color registration error due to a velocity fluctuation of a transfer belt drive motor cannot be resolved even with the above adjustment of the initial rotary phases of the four color photoreceptor drums. The problem of image quality degradation remains unsolved.
Moreover, another problem is that a drive gear of a transfer unit may become eccentric when a transfer unit driver which rotates the transfer unit at a constant velocity and one of the photoreceptors are concurrently driven. This causes a fluctuation velocity in the one photoreceptor and a color shift between a toner image formed on the photoreceptor and toner images formed on the other photoreceptors, degrading image quality.